Unit After studying this Unit, you will be able to explain the terms : system and surroundings; discriminate between close, open and isolated systems; explain internal energy, work and heat; state first law of thermodynamics and express it mathematically; calculate energy changes as work and heat contributions in chemical systems; explain state functions: U , H . correlate ∆ U and ∆ H ; measure experimentally ∆ U and ∆ H ; define standard states for ∆ H ; calculate enthalpy changes for various types of reactions; state and apply Hess’s law of constant heat summation; differentiate between extensive and intensive properties; define spontaneous and non- spontaneous processes; e x p l a i n e n t r o p y a s a thermodynamic state function and apply it for spontaneity; explain Gibbs energy change ( ∆ G ); and establish relationship between ∆ G and spontaneity, ∆ G and equilibrium constant. . Thermodynamic terms We are interested in chemical reactions and the energy changes accompanying them.
For this we need to know certain thermodynamic terms. These are discussed below. . .
The System and the Surroundings A system in thermodynamics refers to that part of universe in which observations are made and remaining universe constitutes the surroundings. The surroundings include everything other than the system. System and the surroundings together constitute the universe. The universe = The system + The surroundings However, the entire universe other than the system is not affected by the changes taking place in the system.
Therefore, for all practical purposes, the surroundings are that portion of the remaining universe which can interact with the system. Usually, the region of space in the neighbourhood of the system constitutes its surroundings. For example, if we are studying the reaction between two substances A and B kept in a beaker, the beaker containing the reaction mixture is the system and the room where the beaker is kept is the surroundings (Fig. .
). Fig. . System and the surroundings the system from the surroundings is called boundary .
This is designed to allow us to